This invention relates to the testing of materials. In particular, this invention comprises an electrochemical cell that is useful for making field measurements of samples of stainless steel to see if the samples have been sensitized.
A problem that is well known to those who work with stainless steel is that of sensitization. Sensitization is a localized loss of the anti-corrosive properties of the stainless steel, either in its manufacture or in fabrication by welding. Stainless steels having chromium concentrations in the range of 18%, such as types 304, 316, and the like, are sensitized when heating to temperatures of 800.degree.-1200.degree. K. causes a formation of the compound chromium carbide in the stainless steel. Both chromium and carbon move within the stainless steel by diffusion, but the carbon moves much faster. Chromium carbide is formed preferentially at grain boundaries, combining with the chromium there to leave localized regions of a mixture with reduced concentrations of free chromium. If the localized regions are reduced to concentrations of the order of 12% or less, the anti-corrosive properties of the stainless steel are affected markedly, and the stainless steel is rendered subject to corrosion and cracking. This is a matter of great concern in stainless steels that are used for piping and the like in nuclear reactors. Sensitization has been observed in some stainless steel pipes that have cracked in service, and sensitization is believed to be the major cause of this cracking. It is desirable to have a non-destructive method of testing 304, 316, or similar high-chromium stainless steels in place to assure that the installed piece has not been sensitized.
It is an object of the present invention to provide an electrochemical cell for field testing of stainless steels for sensitization. Other objects will become apparent in the course of a detailed description of the invention.